PET imaging of protein synthesis rate
Protein synthesis rate (PSR) can be quantified in vivo with labeled L-amino-acids or amino-acid analogues, using PET and adequate analysis methods.
Different proteins contain amino-acids in different proportions, and the relative rates of incorporation into proteins and other metabolic pathways are also variable. Therefore different amino-acid tracers will provide different estimates of PSR. From PET image data it is not possible to discriminate the protein-bound radioactivity from radioactivity bound to other molecules, and therefore the optimal amino acid tracer should have minimal metabolism to non-proteins (Vaalburg et al., 1992, Paans et al., 1996). Production of extracellular matrix proteins can be an important driver for increased amino acid uptake during inflammation and tissue remodelling.
Labelling amino acids with 11C in the carboxylic position increases the specificity of net uptake to PSR, because metabolism through decarboxylation leads to [11C]CO2, which in turn is rapidly cleared from the tissue (Phelps et al., 1984). If amino-acid is labeled in the methyl group, the methyl group may be transferred to many small- and large molecular weight molecules, especially in case of L-[methyl-11C]methionine (Phelps et al., 1984; Ishiwata et al., 1988a). Indeed, the uptake of L-[methyl-11C]methionine and L-2-[18F]fluorotyrosine have been shown to represent mostly amino-acid transport or phospholipid synthesis rather than protein synthesis rate, while L-[1-11C]leucine and L-[1-11C]tyrosine represent PSR (Ishiwata et al., 1988b, 1993 and 1996). Careful modelling may still provide reliable PSR in skeletal muscle with L-[methyl-11C]methionine (Hsu et al., 1996; Fischman et al., 1998).
Transport of amino acids into cells is usually not the rate-limiting step in peripheral tissues and tumours, but in the brain the uptake may be limited by the capacity of the carrier system or partial saturation by physiological amino-acids in plasma (Vaalburg et al., 1992). L-alanine, L-tyrosine, and L-methionine have the highest brain uptake indices (Vaalburg et al., 1992).
In oncological studies the main aim is usually not to measure protein synthesis per se, but cell growth in general. Incorporation of labeled amino-acid in other metabolic pathways than protein synthesis is not a drawback in such studies (Paans et al., 1996). 18F-labelled glutamine analogues are not metabolized in the TCA cycle, but are incorporated into proteins (Yang et al., 2017), and can be used assess transporter (ASCT2) activity or protein synthesis rate.
Bishu S, Schmidt KC, Burlin T, Channing M, Conant S, Huang T, Liu ZH, Qin M, Unterman A, Xia Z, Zametkin A, Herscovitch P, Smith CB. Regional rates of cerebral protein synthesis measured with L-[1-11C]leucine and PET in conscious, young adult men: normal values, variability, and reproducibility. J Cereb Blood Flow Metab. 2008; 28(8): 1502-1513. doi: 10.1038/jcbfm.2008.43.
Crippa F, Alessi A, Serafini GL. PET with radiolabeled aminoacids. Q J Nucl Med Mol Imaging 2012; 56(2): 151-162. PMID: 22617237.
Fischman AJ, Yu YM, Livni E, Babich JW, Young VR, Alpert NM, Tompkins RG. Muscle protein synthesis by positron-emission tomography with L-[methyl-11C]methionine in adult humans. Proc Natl Acad Sci USA 1998; 95: 12793-12798. doi: 10.1073/pnas.95.22.12793.
Hawkins RA, Huang SC, Barrio JR, Keen RE, Feng D, Mazziotta JC, Phelps ME. Estimation of local cerebral protein synthesis rates with L-[1-11C]leucine and PET: methods, model, and results in animals and humans. J Cereb Blood Flow Metab. 1989; 9(4): 446-460. doi: 10.1038/jcbfm.1989.68.
Ishiwata K, Vaalburg W, Elsinga PH, Paans AM, Woldring MG. Comparison of L-[1-11C]methionine and L-methyl-[11C]methionine for measuring in vivo protein synthesis rates with PET. J Nucl Med. 1988a; 29(8): 1419-1427. PMID: 3261334.
Ishiwata K, Vaalburg W, Elsinga PH, Paans AM, Woldring MG. Metabolic studies with L-[1-14C]tyrosine for the investigation of a kinetic model to measure protein synthesis rates with PET. J Nucl Med. 1988b; 29(4): 524-529. PMID: 3258366.
Ishiwata K, Kubota K, Murakami M, Kubota R, Sasaki T, Ishii S, Senda M. Re-evaluation of amino acid PET studies: can the protein synthesis rates in brain and tumor tissues be measured in vivo? J Nucl Med. 1993; 34(11): 1936-1943. PMID: 8229238.
Ishiwata K, Enomoto K, Sasaki T, Elsinga PH, Senda M, Okazumi S, Isono K, Paans AM, Vaalburg W. A feasibility study on L-[1-carbon-11]tyrosine and L-[methyl-carbon-11]methionine to assess liver protein synthesis by PET. J Nucl Med. 1996; 37(2): 279-285. PMID: 8667062.
Mazoyer BM, Heiss WD, Comar D (eds.): PET Studies on Amino Acid Metabolism and Protein Synthesis. Proceedings of a Workshop held in Lyon, France within the framework of the European Community Medical and Public Health Research. Series: Developments in Nuclear Medicine, Vol. 23, 1993, XIV, 270 p. doi: 10.1007/978-94-011-1620-6.
Paans AM, Pruim J, van Waarde A, Willemsen AT, Vaalburg W. Radiolabelled-tyrosine for the measurement of protein synthesis rate in vivo by positron emission tomography. Baillieres Clin Endocrinol Metab. 1996; 10(4): 497-510. doi: 10.1016/S0950-351X(96)80666-5.
Phelps ME, Barrio JR, Huang SC, Keen RE, Chugani H, Mazziotta JC. Criteria for the tracer kinetic measurement of cerebral protein synthesis in humans with positron emission tomography. Ann Neurol. 1984; 15: S192-S202. doi: 10.1002/ana.410150736.
Schmidt KC, Cook MP, Qin M, Kang J, Burlin TV, Smith CB. Measurement of regional rates of cerebral protein synthesis with L-[1-11C]leucine and PET with correction for recycling of tissue amino acids: I. Kinetic modeling approach. J Cereb Blood Flow Metab. 2005; 25(5): 617-628. doi: 10.1038/sj.jcbfm.9600067.
Smith CB, Davidsen L, Deibler G, Patlak C, Petigrew K, Sokoloff L. A method for the determination of local rates of protein synthesis in brain. Trans Am Sot Neurochem. 1980; 11:94.
Smith CB, Deibler GE, Eng N, Schmidt K, Sokoloff L. Measurement of local cerebral protein synthesis in vivo: influence of recycling of amino acids derived from protein degradation. Proc Natl Acad Sci USA 1988; 85(23): 9341-9345.
Smith CB, Schmidt KC, Qin M, Burlin TV, Cook MP, Kang J, Saunders RC, Bacher JD, Carson RE, Channing MA, Eckelman WC, Herscovitch P, Laverman P, Vuong BK. Measurement of regional rates of cerebral protein synthesis with L-[1-11C]leucine and PET with correction for recycling of tissue amino acids: II. Validation in rhesus monkeys. J Cereb Blood Flow Metab. 2005; 25(5): 629-640. doi: 10.1038/sj.jcbfm.9600066.
Sundaram SK, Muzik O, Chugani DC, Mu F, Mangner TJ, Chugani HT. Quantification of protein synthesis in the human brain using L-[1-11C]-leucine PET: incorporation of factors for large neutral amino acids in plasma and for amino acids recycled from tissue. J Nucl Med. 2006; 47(11): 1787-1795.
Tomasi G, Bertoldo A, Bishu S, Unterman A, Smith CB, Schmidt KC. Voxel-based estimation of kinetic model parameters of the L-[1-11C]leucine PET method for determination of regional rates of cerebral protein synthesis: validation and comparison with region-of-interest-based methods. J Cereb Blood Flow Metab. 2009; 29(7): 1317-1331. doi: 10.1038/jcbfm.2009.52.
Vaalburg W, Coenen HH, Crouzel C, Elsinga PH, Långström B, Lemaire C, Meyer GJ. Amino acids for the measurement of protein synthesis in vivo by PET. Int J Rad Appl Instrum B. 1992; 19(2): 227-237. doi: 10.1016/0883-2897(92)90011-M.
Veronese M, Schmidt KC, Smith CB, Bertoldo A. Use of spectral analysis with iterative filter for voxelwise determination of regional rates of cerebral protein synthesis with L-[1-11C]leucine PET. J Cereb Blood Flow Metab. 2012; 32(6): 1073-1085. doi: 10.1038/jcbfm.2012.27.
Updated at: 2019-01-04
Created at: 2015-01-26
Written by: Vesa Oikonen